Steel compositions, methods of manufacture and uses in producing rimfire cartridges

ABSTRACT

The present invention relates generally to steel compositions, methods of manufacturing the compositions and using the compositions to produce rimfire ammunition cartridges. The steel compositions for use in the rimfire cartridges are processed through cold-rolling and annealing steps to create suitable physical properties.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application claims the benefit of U.S. Provisional PatentApplication No. 62/092,359, entitled “Steel Compositions, Methods ofManufacture and Uses in Producing Rimfire Cartridges”, filed on Dec. 16,2014, the contents of which are incorporated herein by reference.

BACKGROUND OF INVENTION Field of Invention

The invention relates generally to steel compositions, methods ofmanufacturing the compositions and uses of the compositions to producerimfire ammunition cartridges.

Description of Related Art

In general, rimfire ammunition cartridges are sufficiently strong towithstand pressures created by ignition of a propellant, while beingsufficiently elastic to permit extraction from the chamber or barrel ofa firing device after firing. Brass has traditionally been used for thistype of ammunition. Its physical properties allow for the manufacture ofrimfire cartridge cases that satisfy the strength and elasticityrequirements. Brass is corrosion resistant, formable and highly elastic.Thus, the use of brass results in little or no problems when extractingthe cartridge from the firearm after firing. Brass work-hardens to anextent that provides adequate strength to withstand the explosive forceof the powder charge with minimal failures of the cartridge sidewalls.It is relatively soft and therefore, can be formed with minimal toolwear in manufacturing. Thus, it has been shown that brass is a preferredmaterial in the manufacture of rimfire ammunition cartridges. However,one disadvantage of brass is its cost; brass is typically expensive andits price has been known to fluctuate significantly.

In developing a less expensive metal as an alternative, steel has beenconsidered as a replacement for brass. Advantageously, stress corrosioncracking and reaction with primers and powders are not problemsassociated with the use of steel. Although, one disadvantage is thatsteel does not have the same elastic recovery as brass. As a result,there are extraction concerns associated with the removal of steelcartridges from the chamber or barrel of the firearm after firing. Forexample, with the use of low carbon steels, such as C1008/1010,extraction problems may be severe due to the elasticity of lower carbonsteel being much less as compared to brass.

FIG. 1 is a plot of stress-strain curves for brass and steel, and shows,elastic strain for steel 1, elastic strain for brass 2, total strain tofailure for steel 3, total strain to failure for brass 4, yield strengthfor brass 5, tensile strength for brass 6, yield strength for steel 7,tensile strength for steel 8, and slope 9. Young's modulus, e.g.,elastic recovery, for brass and steel can be determined based on theslope of each of the stress-strain curves in the elastic region. Theslope of the curve for brass is one-half that of steel, as shown inFIG. 1. Young's modulus for brass is approximately 15×10⁶ psi while themodulus for steel is approximately from 29×10⁶ to 30×10⁶ psi. In FIG. 1,slope 9 is calculated as stress divided by strain in the elastic region.Elastic strain of steel 1 and elastic strain of brass 2 are shown inFIG. 1. The elastic recovery for brass is twice that of steel. Thus,brass has almost twice the elasticity as steel for the equivalent stresslevel. As a result, a brass cartridge when fired will expand in diameterdue to the internal pressure and essentially seal the internal diameterof the chamber. After firing, the brass cartridge will then “shrink” indiameter such that its diameter is less than the internal diameter ofthe chamber and therefore, the cartridge can be easily removed from thechamber.

FIG. 2 is a schematic showing a portion of a firing device, including acartridge head 11 and a cartridge sidewall 12, positioned within achamber 13 of a barrel 14 of the firing device and an extractor 15 foruse in extracting the cartridge 11,12 from the chamber 13 after firingthe firing device. Further, FIG. 2 includes a bolt 10, firing pin 16,and a sidewall 17 sealing the chamber.

Since the elasticity and elastic recovery of steel is significantly lessthan brass, the diameter of a typical low carbon steel cartridge willexpand to seal the chamber upon firing of a firing device; however,after firing, the diameter of the low carbon steel cartridge will shrinkless, e.g., only half as much as brass because (as shown in FIG. 1)brass has almost twice the elasticity of steel. The amount by which thediameter of the steel cartridge shrinks may not be sufficient to allowthe cartridge to be easily extracted from the chamber after firing. As aresult, the cartridge can lodge in the chamber of the barrel of thefiring device.

Additionally, as shown in FIG. 2, one or more sidewall splits 18 (whichis exaggerated) may occur with low carbon steel due to the material,even after forming and work hardening, not being sufficiently strong orductile to withstand the internal explosion experienced by the cartridgeupon firing of the device. Without intending to be bound by anyparticular theory, it is believed that in order for steel to elasticallyrecover to the same extent as brass, the steel should have about twicethe yield strength as brass in the drawn sidewall of the cartridge(after work hardening in forming). However, it is very likely that yieldstrength values lower than twice that of steel (in the cartridgesidewalls) would be sufficient to allow for acceptable extraction uponfiring.

Alternatively, higher carbon steels could be used to increase thestrength of the cartridge to overcome the aforementioned problems;however, there are anticipated problems relating to forming and toolwear, as well as the steel likely being too hard for the firing pin todeform the rim of the cartridge. Generally, rimfire cartridges have rimsthat are deformable by the firing pin as a mechanism to ignite thepriming powder, which is contained within the case of the cartridge.

Heat treating steel cartridge cases, which have already been formed, canreduce tool wear and increase strength. U.S. Pat. No. 2,373,921 to Snelland U.S. Pat. No. 2,698,268 to Lyon disclose a method of forming steelcartridge cases requiring a heat treatment or annealing step after thecase is formed. However, heat treatments on a batch of small parts, likecases for rimfire ammunition, does not produce uniform results throughall of the parts. Unlike Snell or Lyon, the steel rimfire cartridge ofthe present invention requires no further treatments after the case isformed. In addition, neither Snell nor Lyon contemplates the use ofsteel cases formed by their methods for use in rimfire ammunition, butrather apply the invention to the production of center fire ammunition.

Thus, there is a need in the art to design and develop a metal or metalalloy for use in manufacturing rimfire ammunition cartridges that is areplacement for the typical brass material that is known in the art.

SUMMARY OF INVENTION

The present invention relates generally to steel compositions andmethods of processing the steel compositions for producing steel-casedrimfire cartridges. In one aspect, the invention provides a steelcomposition for rimfire ammunition cartridges. The composition includesfrom about 0.03 to about 0.18 weight percent carbon, from about 0.15 toabout 1.60 weight percent silicon, from about 0.60 to about 2.50 weightpercent manganese, from greater than 0 to about 0.025 weight percentphosphorus, from greater than 0 to about 0.025 weight percent sulfur andfrom about 0.20 to about 0.08 weight percent aluminum, based on thetotal weight percent of the composition.

The composition can further include one or more metal elements selectedfrom the group consisting of cobalt, columbium, chromium, copper,molybdenum, nickel, titanium, vanadium, zirconium and mixtures andalloys thereof. The one or more of the metal elements present in thecomposition can constitute typically no more than about 0.22 weightpercent, based on total weight of the composition.

In certain embodiments, the composition can include from about 0.05 toabout 0.13 weight percent carbon, from about 0.15 to about 0.50 weightpercent silicon, from about 0.70 to about 2.50 weight percent manganese,about 0.025 weight percent phosphorus, about 0.025 percent sulfur, fromabout 0.20 to about 0.08 weight percent aluminum and less than about0.22 weight percent of the one or more metal elements, based on thetotal weight of the composition.

In certain other embodiments, the composition can include from about0.16 to about 0.18 weight percent carbon, from about 1.25 to about 1.55weight percent silicon, from about 1.9 to about 2.1 weight percentmanganese, about 0.02 weight percent phosphorus, about 0.02 percentsulfur, from about 0.025 to about 0.055 weight percent aluminum, lessthan about 0.06 weight percent copper, less than about 0.04 weightpercent nickel, less than about 0.06 weight percent chromium and lessthan about 0.02 weight percent molybdenum, based on the total weight ofthe composition.

In certain other embodiments, the composition can include from about0.126 to about 0.154 weight percent carbon, from about 0.395 to about0.605 weight percent silicon, from about 1.75 to about 1.95 weightpercent manganese, about 0.02 weight percent of phosphorus, about 0.005percent sulfur, from about 0.02 to about 0.06 weight percent aluminum,less than about 0.06 weight percent copper, less than about 0.04 weightpercent nickel, less than about 0.06 weight percent chromium and lessthan about 0.02 weight percent molybdenum, based on the total weight ofthe composition.

In another aspect, the invention provides a method of processing a steelcomposition for a rimfire cartridge. The method includes obtaining asteel composition having an original thickness, cold rolling the steelcomposition to reduce the original thickness by at least 70%, to producea cold rolled steel composition having an intermediate thickness,performing a first annealing and subsequent cooling of the steelcomposition with an intermediate thickness to produce an annealedintermediate steel composition, cold rolling the annealed intermediatesteel composition to a thickness that is reduced about 20% to about 35%from the intermediate thickness of the intermediate steel composition toproduce a steel composition having a final thickness, performing asecond annealing and subsequent cooling of the steel composition havinga final thickness to produce a final annealed steel composition having afinal thickness, and continuous plating the final annealed steelmaterial having a final thickness.

In certain embodiments, the first and second annealing steps areconducted as a batch process. In other embodiments, the first and secondannealing steps are conducted as a continuous process.

In certain embodiments, the continuous plating step can be performedprior to the second cold rolling step. This continuous plating step canbe in addition to or in place of the continuous plating step performedafter the second annealing and cooling step. The continuous plating caninclude zinc, brass or combinations and alloys thereof.

The steel composition obtained can have an original thickness of about0.090 inches. Further, the steel composition obtained can be in a formselected from hot roll, hot roll that is pickled and oiled, and dualphase cold roll. The steel composition obtained can be at leastpartially reduced such that the reduction in the first cold rolling stepmay be modified or eliminated. In certain embodiments, the steelcomposition obtained is an intermediate cold rolled composition. Inother embodiments, the steel composition obtained is a dual phase coldrolled composition. In these embodiments, an initial annealing andcooling step is performed prior to the first cold rolling step.

The method of forming the rimfire cartridge can be selected from a cup,draw and head process, and a progressive die and head process.

In another aspect, the invention provides a method of processing a steelcomposition to form a rimfire cartridge. The method includes obtaining asteel composition having an original thickness, cold rolling the steelcomposition to produce a steel composition having a final thickness,annealing and subsequently cooling the steel composition having a finalthickness to produce a final annealed steel composition having a finalthickness, and continuous plating the final annealed steel compositionhaving a final thickness.

The rimfire cartridge can include a case composed of the steelcomposition recited above, having a first end and a second end, a rimformed on the first end of the case, a projectile pressed into thesecond end of the steel case, a priming compound contained within therim, and a propellant contained within the case.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot showing a stress/strain curve for each of brass andsteel.

FIG. 2 is a schematic showing a rimfire ammunition cartridge inside achamber of a firing device.

FIGS. 3A, 3B and 3C generally show a batch process in accordance withthe invention that is used to treat or process steel and form it into arimfire cartridge case.

FIG. 4 is a flow diagram for a continuous Process Route #1 and acontinuous Process Route #2 in accordance with the invention that isused to treat or process steel and form it into a rimfire cartridgecase.

DETAILED DESCRIPTION OF THE INVENTION

Rimfire cartridges are generally known in the art and, they aretypically composed of brass and manufactured by conventional methodsemployed for processing the cartridges. The methods include the cup,draw and head method, or, alternatively, the progressive die and headmethod. The invention provides steel compositions as replacements forbrass in the manufacture of rimfire ammunition cartridges and, inparticular, for use in .22 caliber firing devices. The invention alsoprovides methods of processing and treating, e.g., annealing, the steelcompositions such that they produce rimfire cartridges that demonstrateone or more of the following properties:

-   -   relatively soft in the rim such that the firing pin will deform        the material enough to ignite the primer upon firing of a firing        device;    -   work-hardened to a level approaching twice that of typical brass        (which may be somewhat less in the batch annealed product) in        order to achieve sufficient elastic recovery to prevent problems        associated with extracting the cartridge from the chamber in the        barrel of the firing device;    -   formable enough to produce the cartridge; and    -   sufficiently strong and ductile to reduce or preclude failures        of the cartridge side walls, e.g., splits, upon firing of the        firing device.

The steel compositions produced in accordance with the invention canvary and can depend on the particular steel manufacturer and the amountof alloy components employed. In certain embodiments, the steelcompositions include from about 0.03 to about 0.18 weight percentcarbon, from about 0.15 to about 1.60 weight percent silicon, from about0.60 to about 2.50 weight percent manganese, from greater than 0 toabout 0.025 weight percent phosphorus, from greater than 0 to about0.025 weight percent sulfur, and from about 0.02 to about 0.08 weightpercent aluminum, based on the total weight percent of the composition.In addition, the compositions may include one or more metal elementsselected from the group consisting of cobalt, columbium, chromium,copper, molybdenum, nickel, titanium, vanadium, zirconium and mixturesand alloys thereof. In certain embodiments, when one or more of thesemetal elements are present in the compositions, the one or more metalelements typically constitute no more than about 0.22 weight percent,based on the total weight of the composition.

In certain embodiments, the composition can include from about 0.05 toabout 0.13 weight percent carbon, from about 0.15 to about 0.50 weightpercent silicon, from about 0.70 to about 2.50 weight percent manganese,about 0.025 weight percent phosphorus, about 0.025 percent sulfur, fromabout 0.20 to about 0.08 weight percent aluminum and less than about0.22 weight percent of the one or more metal elements, based on thetotal weight of the composition.

In certain other embodiments, the composition can include from about0.16 to about 0.18 weight percent carbon, from about 1.25 to about 1.55weight percent silicon, from about 1.9 to about 2.1 weight percentmanganese, about 0.02 weight percent phosphorus, about 0.02 percentsulfur, from about 0.025 to about 0.055 weight percent aluminum, lessthan about 0.06 weight percent copper, less than about 0.04 weightpercent nickel, less than about 0.06 weight percent chromium and lessthan about 0.02 weight percent molybdenum, based on the total weight ofthe composition.

In certain other embodiments, the composition can include from about0.126 to about 0.154 weight percent carbon, from about 0.395 to about0.605 weight percent silicon, from about 1.75 to about 1.95 weightpercent manganese, about 0.02 weight percent of phosphorus, about 0.005percent sulfur, from about 0.02 to about 0.06 weight percent aluminum,less than about 0.06 weight percent copper, less than about 0.04 weightpercent nickel, less than about 0.06 weight percent chromium and lessthan about 0.02 weight percent molybdenum, based on the total weight ofthe composition.

In accordance with the invention, the steel compositions undergo variousprocessing steps to provide steel materials that are suitable for use informing rimfire ammunition cartridges. The steel compositions can beobtained or received from a producer in various forms that are known inthe art. For example, the steel composition can be received in a hotrolled condition, either as black band (with scale intact; to be pickledor de-scaled, for example, by a customer) or in a pickled and oiledcondition. The steel in the hot rolled condition is processed on a hotstrip mill which can result in the following initial mechanicalproperties, for example: about 80 KSI minimum yield strength, about 95KSI minimum tensile strength and about 10% minimum elongation in 2″.Alternatively, the steel as received from the producer can have about110 KSI yield strength, about 113 KSI tensile strength and about 16%elongation in 2″. These steels are commonly used in the hot rolled stateand thus, the as-received mechanical properties are desired by the enduser, e.g., customer. In certain embodiments, there may be no propertiesguaranteed or cited by the hot roll producer since the end user will begenerating the final properties by its own processing, which includescold rolling and annealing. Typically, the end user may specify generalhot roll parameters relating to finishing and cooling temperatures, suchas “high finishing” and “low cooling” temperatures. In otherembodiments, steel can be received from the producer in an intermediategage versus hot rolled. In these embodiments, the steel can be suppliedeither as cold rolled (hard temper), regular annealed, or dual phaseannealed (or other higher strength structure). Intermediate gage steelmay be used to shorten the processing steps (designed to fit into thesubsequent processing scheme), or to utilize the intermediateproperties. Dual phase intermediate steels can have various mechanicalproperties. In certain embodiments, the dual phase steel can have about130.8 KSI yield strength, about 165.9 KSI tensile strength and about10.6% elongation in 2″, or about 154 KSI yield strength, about 182 KSItensile strength and about 9.9% elongation in 2″. In any case, the steelas received from the producer is then processed to a finished annealedproduct for use making rim fire ammunition cartridges.

The processing of the steel includes annealing, which can be conductedby employing a batch process or a continuous process. The steel asreceived (from a producer) and being processed can have the beginningtypical mechanical properties, as mentioned above, and is pretreated bythe removal of scale and cutting to a width that allows for subsequentprocessing. FIGS. 3A, 3B and 3C are schematics that show typical stepscarried out in the batch annealing process of the steel composition. Theprocess steps in FIGS. 3A and 3B assume that the steel composition asreceived from the producer has not been processed. That is, the steelcomposition is in the form of an unprocessed hot roll or hot roll in apickled and oiled condition. However, it is contemplated and understoodthat the steel composition as received from the producer may have beenprocessed to an extent and therefore, can be in the form of a partiallyprocessed roll. In this situation wherein partial processing has beendone by the producer of the received roll, it may be appropriate tomodify or eliminate a step in the processes as shown in FIGS. 3A and 3B.For example, the processes shown in FIGS. 3A and 3B begin with pickling(de-scaling) 20 the roll. If the roll is received in a pickledcondition, this initial step can be skipped or eliminated. Further, thenext step identified in FIGS. 3A and 3B is cold rolling 22 the steelcomposition to an intermediate thickness that constitutes a reduction ofabout 70% from the initial thickness of the steel composition. If thesteel composition received from the producer, e.g., the starting steelcomposition, has already been partially processed, it may not benecessary to perform a cold roll reduction of 70%. Thus, this initialcold rolling step 22 can be modified or even eliminated to accommodatethe properties and extent of processing of the received steel roll. Inone embodiment, the received steel roll can be in the form of a dualphase cold roll which is partially processed such that the cold rollstep can be performed to reduce the thickness by about 30% as a minimum(instead of about 70% as recited in FIGS. 3A and 3B). Further, withrespect to a received steel roll that is in the form of a dual phase, itmay be appropriate to perform an anneal, e.g., batch anneal, prior tothe initial cold roll step. This situation involving a received dualphase cold roll is shown in FIG. 3C, which is described later herein.

In accordance with FIGS. 3A and 3B, following the initial cold roll step22, the steel composition having the intermediate thickness is subjectedto an initial or first batch annealing 24 and subsequent cooling 26. InFIG. 3A, these batch annealing and cooling steps 24,26 are followed bycontinuous plating 28. However, in FIG. 3B, the continuous plating 28 isnot conducted until the end of the process, e.g., following a second orfinal batch annealing 32 and cooling 34 process. It is contemplated andunderstood that the continuous plating 28 can be conducted according toeither of the processes recited in FIGS. 3A and 3B, and additionally,the continuous plating 28 can be conducted according to both of theprocesses of FIGS. 3A and 3B. That is, the continuous plating 28 can beconducted following the initial or first batch annealing 24 andsubsequent cooling 26 process, and following the final or second batchannealing 32 and subsequent cooling 34 process. The continuous plating28 includes applying or depositing a coating composition to form a layeror coating thereon. The coating can include elemental zinc or zincalloy, elemental brass or brass alloy, or other protective coating.

Following, the first batch annealing and cooling 24,26 (and optionallyplating 28), the steel composition is subsequently subjected to a secondcold roll process 30 that reduces the thickness an additional 20-35% toobtain a final thickness, followed by a second or final batch annealing32 and cooling 34, for producing rim fire cartridges 36 in accordancewith conventional techniques.

In certain embodiments, the batch annealing consists of heating thesteel to a temperature of about 500° F. for a period of about 0.5 hours.The temperature is then increased to a temperature of about 1,250° F.over a period of 8.5 hours and subsequently, the temperature isincreased to a temperature of about 1,300° F. over a period of 1.5 hoursand held at this temperature for about 6.0 hours. The steel is thencooled to ambient temperature and plated. Following continuous plating,the steel is further processed by cold rolling to a final thickness,which provides about 20 to about 35% further reduction. Withoutintending to be bound by any particular theory, it has been found thatlimiting the additional reduction to the range from about 20 to about35% produces steel with different physical properties than are typicalfor the grade of steel. The steel having the final thickness is batchannealed, cooled and then formed into rimfire cartridges.

The batch process described in FIG. 3C shows typical processing stepsfor using intermediate thickness steel 40, either in the cold rolledstate, or dual phase annealed state. FIG. 3C includes an initial orfirst batch annealing and cooling 42, followed by cold rolling 44 to anintermediate thickness (about 30-70% of the initial thickness), and thenanother or second batch annealing and cooling 46. Subsequently, a secondcold rolling 48 is conducted to provide a final thickness (about anadditional 20-35% reduction), followed by another or final batchannealing and cooling 50. As shown in FIG. 3C, continuous plating 52 ofthe final annealed steel composition is performed such that the steelcomposition is suitable for use in forming a rimfire cartridge 54. InFIG. 3C, continuous plating is shown as the last step before forming thecartridges. However, as in FIG. 3A, an option is to perform continuousplating 52 after the second batch annealing 46, then cold rolling 48 toa final thickness and subsequently performing the final batch annealing50.

The processes employed to form the rimfire cartridges can includeconventional apparatus and methods known in the art, such as, but notlimited to, cup, draw and head processes and progressive die and headprocesses.

It should be noted that for the processes shown in FIGS. 3A, 3B and 3C,processing can also include rolling directly to gage with nointermediate anneal. In so doing, the propensity for earing is aconsideration. If directly rolling from hot band, a high degree ofreduction (e.g., in the range of from about 85 to about 88%) maycontribute to minimizing earing, depending on the chemistry. Thisalternative or option is equally applicable to FIG. 4, which isdescribed below.

FIG. 4 is a schematic showing the steps that can be conducted in acontinuous processing, e.g., annealing, of a steel composition. FIG. 4identifies a Process Route #1 and a Process Route #2. As shown in FIG.4, Process Route #1 includes starting with hot rolled high strengthsteel having a thickness of about 0.090 inch. If the hot roll is notreceived in a pickled condition, pickling or de-scaling 60 is conductedto produce the pickled hot roll 62 (as identified in FIG. 4 as theinitial step). The pickled hot roll 62 is cold rolled 64 to a finishedgage or thickness and then, continuous annealing 66 and subsequent rapidcooling 68 are performed, followed by continuous plating 70 with zinc,brass or other protective coating. The resulting processed steelcomposition is then used to produce rimfire cartridges 72 formed usingconventional techniques. Further, as shown in FIG. 4, Process Route #2includes starting with a pickled hot rolled high strength steel 60,62having a thickness of about 0.090 inch, as shown in Process Route #1.Further, Process Route #2 includes cold rolling 64A to an intermediatethickness, followed by intermediate continuous annealing 66 and rapidcooling 68, followed by further cold rolling 74 to a final thicknesswhich provides for about 20-35% reduction and then, continuous annealing76 and rapid cooling 78, followed by continuous plating 70 with zinc,brass or other protective coating. The resulting processed steelcomposition is then used to produce rimfire cartridges 72 formed usingconventional techniques.

In Process Route #1, the temperature of the continuous anneal istypically about 1775° F. The subsequent cool is for about 1-2 minutes toabout room temperature, before recoiling at the exit end of thecontinuous anneal furnace. For the steel composition that includes fromabout 0.05 to about 0.13 weight percent carbon, from about 0.15 to about0.50 weight percent silicon, from about 0.70 to about 2.50 weightpercent manganese, about 0.025 weight percent phosphorus, about 0.025percent sulfur, from about 0.02 to about 0.08 weight percent aluminumand less than about 0.22 weight percent of the one or more metalelements, based on the total weight of the composition, the continuousannealed structure produced by Process Route #1 (as shown in FIG. 4) canbe comprised of very fine-grained ferrite (ASTM #12-14 approximately),residual carbide particles containing small amounts of V and Cb, and asmall volume fraction of martensite in pools formed from partialaustenitization at high temperature (and fast cooling in the continuousanneal). The combination of solid solution strengthening from high Mnand Si contents, the very fine grain size, the precipitation hardeningeffect of the carbide particles, and the presence of the martensiticsecond phase, all contribute to producing a steel with a very highwork-hardening rate, yet relatively low yield strength (comparable toregular low carbon 1008/1010 steel). Typical tensile properties of thissteel, cold rolled and heat treated as described above, are as follows:

Yield Strength—40-50 KSI

Tensile Strength—80-100 KSI (typically 88-95 KSI)

Elongation—20-30%

The fact that the tensile strength is almost twice as high as the yieldstrength, indicates the high work-hardening characteristics of thismaterial. Annealed brass generally shows the same effect, with thetensile strength being about 2 to 2½ times that of the yield strength.This material, when severely ironed in the cartridge sidewall, willproduce the high yield strength needed for sufficient elastic recovery(successful extraction) and high enough strength to prevent sidewallsplits.

In Process Route #2, the steel is continuously annealed in the rangefrom about 1400 to about 1775° F. to soften the final cold rolling. Thefinal anneal is the same as for Process Route #1, i.e., continuousannealing at about 1775° F. and rapid cooling to approximately roomtemperature (1-2 minutes) and recoiling at the exit end of the furnace.Process Route #2 can produce a structure virtually identical to thestructure produced in Process Route #1 but yields some advantages asfurther described. Steel processed in the Process Route #1 typicallyshows a significant degree of earing or planar anistropy (due topreferred crystal orientation). Earing can result in slight non-uniformthickness around the circumference of the cartridge. By using theintermediate anneal step and final thickness reduction in Process Route#2, this earing tendency can be reduced or minimized. In addition, thisstep may also tend to keep the yield strength toward the low side of therange because of a very slight coarsening of the ferrite grain size.Even though both Process Routes #1 and #2 are effective to produce steelthat functions well for rimfire cartridges, steel processed or treatedby Process Route #2 may tend to form more consistently due to theminimized earing and slightly less thickness variation around thecartridge circumference.

In certain embodiments, the invention provides treatment of a highstrength low alloy steel which includes continuous annealing at hightemperatures to produce a dual phase steel. In certain otherembodiments, the invention provides treatment of grade 409 or 410stainless steel, which includes batch processing at lower temperatures.Use of these materials are advantageous in that no corrosion-resistantcoating is needed following treatment of the steel.

The treated steel can be produced to minimum earing properties, e.g.,similar to the processing of brass. Without intending to be bound by anyparticular theory, it is believed that this allows for easiermanufacturing and more uniformity in the cartridge wall. Further, acoarsening of the grain size may occur, which will produce lower yieldstrength, e.g., to facilitate rim firing, and a slightly higher workhardening rate when the steel is formed into a cartridge.

The steel can be pre-plated for corrosion resistance to provide extralubrication for the drawing process, and to minimize tooling wear. Bothbrass plate and zinc plate with a special clear chromate (for extraprotection from white rust with the zinc coat) can be used. Otherplating types such as copper, cadmium, nickel, nickel-zinc, or anyothers which could provide lubrication and are drawable enough forforming, also can be used. However, some of these may be prohibitivelyexpensive.

In certain embodiments, the steel can be plated prior to forming toprovide extra lubrication and to reduce tool wear. Alternately, thesteel can be plated for corrosion resistance after the ammunition casesare formed.

One having skill in the art will appreciate that the specifications ofthe annealing process, including the time periods and temperatures, canvary depending on the type of equipment used, among other factors.Alternative annealing parameters may be used that achieve the sameresult.

EXAMPLES

Testing of Mechanical Properties for Brass and Steel

The mechanical properties in the wall of an existing ammunitioncartridge made from brass were determined in order to identify theproperties a steel composition should possess to match, or closelyapproximate, the elastic recovery of brass. Rimfire cartridges arebasically drawn and ironed, with mostly ironing in the final formingstages. Since it was essentially impossible to sample the cartridge walland measure tensile properties, a method to approximate this wasnecessary. Ironing most closely resembles cold rolling (for the simplestmechanical working process). Sections of cartridge walls were measuredfor thickness and then, annealed brass strips were cold rolled to thesame thickness (the brass cartridge walls after drawing and ironingranged from about 0.012″ to 0.008″ from near the cartridge base to theopen end and therefore, the same thickness was used for rolling thestrips). Since the brass used for rimfire cartridges typically is about0.020″ in thickness, depending on the producer, low carbon steel and thebrass plated strips were originally rolled from about 0.020″ material.All of the brass and baseline low carbon steel strips were in thenormally-used annealed condition (the brass was obtained directly from acartridge manufacturer, and the steel was standard 1008/1010 low carbonbatch annealed steel).

Tensile tests were performed on the cold rolled brass and low carbonsteel strips to determine the yield strength, tensile strength andelongation. The results, comparing the brass to the low carbon steelproperties, were used to develop special high strength steels having thefollowing compositions: from about 0.03 to about 0.18 weight percentcarbon, from about 0.15 to about 1.60 weight percent silicon, from about0.60 to about 2.50 weight percent manganese, from greater than 0 toabout 0.025 weight percent phosphorus, from greater than 0 to about0.025 weight percent sulfur and from about 0.20 to about 0.08 weightpercent aluminum and less than about 0.22 weight percent of the one ormore metal elements, based on the total weight of the composition.

After processing the trials of the high strength steels, strip sampleswere taken and cold rolled to the same, or approximate thicknesses asthe brass and low carbon steel for comparison purposes. The results ofthe cold rolling experiments are shown below in Table 1. Only the yieldstrengths (in KSI) are shown because this is what was used to determinethe elastic recovery.

TABLE 1 Material Annealed @0.012″ @0.10″ @0.008″ Cartridge Brass 21.078.8 83.5 89.8 Reg. Low Carbon 48.0 88.1 91.5 95.7 High Strength, 48.0134.6 140.3 152.1 Continuous Annealed High Strength, #1 44.2 115.3*131.8 143.7** Batch Annealed High Strength, #2 31.8 86.0*** 105.6 ****Batch Annealed *Actual thickness was 0.0134″ **Actual thickness was0.0094″ ***Actual thickness was 0.01275″ **** Rolling mill could notachieve 0.008″

Table 1 shows that yield strength in the brass sidewalls ranged fromabout 79 to about 90 KSI, while that of the regular low carbon steel wasabout the same. Due to the difference in elastic modulus between brassand steel, this means that the low carbon steel would “shrink” back halfas much as the brass and this is why extraction problems occur whenordinary low carbon steel is used to make cartridges. However, thecontinuously annealed high strength steel as shown in the Table 1,work-hardens to a range of about 135 to about 142 KSI. Even though thisis slightly less than twice that of the brass numbers, test firings ofcartridges made of this steel at the manufacturer showed no extractionproblems in several gun types including revolvers and semi-automaticpistols. One objective method used to test extraction by themanufacturer, is to fire six cartridges in a revolver and then measurethe actual force it takes to “push” all six cartridges out using tomanual extractor rod. This was done with some cartridges made from thecontinuous annealed steel. Brass was fired first for a baseline and inthis firing of three gun loads, the results averaged 2.2 lb. force (anempty gun pushed at about 1.5 lb. force). The steel cartridges averagedunder 2 lb. force and there were no misfires or sidewall splits.Although, there were a few misfires in some of the other gun types.Thus, in revolver tests, the continuously annealed steel cartridgesperformed as good as brass. Misfires may occur because the yieldstrength of brass is significantly less than for the steel. Thus, thefiring pin deforms the steel to a lesser extent than the brass andtranslated less deformation to the primer. To minimize this effect, theprimer mixture can be adjusted slightly for sensitivity to deformation,or the steel may be made slightly thinner to allow for more deformationto occur. These rimfire cartridges were made from the cup, draw, andhead method, as well as the progressive die and head method, and werezinc-plated.

The results shown in Table 1 for the continuous annealed high strengthsteel and firing tests with cartridges made from this steel, wereobtained by heat treating the steel in accordance with FIG. 4, ProcessRoute #2. This steel has the following composition: from about 0.05 toabout 0.10 carbon, from about 0.20 to about 0.50 silicon, less thanabout 0.07 chromium, from about 0.70 to about 1.45 manganese, from about0.05 to about 0.14 vanadium, less than about 0.05 nickel, less thanabout 0.02 phosphorus, from about 0.04 to about 0.12 columbium, lessthan about 0.03 molybdenum, less than about 0.016 sulfur, from about0.02 to about 0.08 aluminum, based on the total weight of thecomposition.

Table 1 shows that High Strength #1, Batch Annealed steel work hardenedto levels of about 115 KSI to about 144 KSI, similar to the continuousannealed high strength steel. Yield strength was also close to thecontinuously annealed steel at about 44 KSI. Rimfire cartridges weremanufactured successfully from this steel at a rim fire manufacturingplant and test fired. The same revolver test as previously discussedabove was used with these cartridges also. There were significantsidewall splits with some of these cartridges. Upon examination, itappeared that the cracks were emanating from die scratches all aroundthe tops of the cartridges. It should be noted that this steel wassupplied in test quantity and was not plated. When the splits did notinhibit the extractor “push”, levels of about 2.2 lb. extraction forcewere measured. Thus, the level of strength in the sidewalls of thecartridges made from this steel, are enough to allow good extraction. Itis anticipated that the plating will result in much better lubricationin the draw dies, and this should eliminate the tendency for diescratches. The lubrication normally used for brass, was probably notoptimal for bare steel. There were also some misfires in this testing.The discussion of this in the above paragraph also applies with thismaterial. These cartridges were made from the cup, draw and head method.

The results shown in Table 1 for High Strength #1, Batch Annealed steeland firing tests with cartridges made from this steel, were obtained byheat treating the steel in accordance with FIG. 3C. This steel has thefollowing composition: about 0.16 to about 0.18 percent carbon, fromabout 1.25 to about 1.55 percent silicon, from about 1.9 to about 2.1percent manganese, about 0.02 percent phosphorus, about 0.02 percentsulfur, about 0.025 to about 0.055 percent aluminum, less than about0.06 percent copper, less than about 0.04 percent nickel, less thanabout 0.06 percent chromium and less than about 0.02 percent molybdenum,based on the total weight of the composition.

Table 1 shows that High Strength #2, batch annealed steel work hardenedto levels of about 86 KSI to 106 KSI, lower than for High Strength #1,Batch Annealed steel and High Strength Continuous Annealed steel.However, it is noted that the highest level here was for material at0.010″ and not 0.008″; the rolling mill available to roll this materialcould not achieve 0.008″. Extrapolating the data would put the yield at0.008″ at approximately 115-120 KSI. Also, part of the reason for thesomewhat lower values is the lower silicon level for High Strength #2,Batch Annealed steel. This chemistry was used in order to obtain a lowervalue to attempt to determine the lowest yield strength capable ofproducing cartridges that would extract acceptably, and provide lesstendency for misfires (due to the lower as-annealed yield strength).Rimfire cartridges were manufactured successfully from this steel,although a small amount of earing was present. Firing tests with thisHigh Strength #2, Batch Annealed steel were conducted concurrently withthe firing tests for High Strength #1 material described above. Thebrass test firing baseline was 2.2 lb. force for 3 gun loads. Theresults for High Strength #2, Batch Annealed steel averaged 2.46 lb.force and there were no misfires, so this material was a (perfect) matchfor the brass. Thus, it has been determined that the yield strengthneeded in the sidewalls of the rimfire cartridge to allow goodextraction, can be significantly lower than twice the level of brass,but these levels can only be achieved with specially selected andprocessed steels. These cartridges were made by the cup, draw and headmethod.

The results shown in Table 1 for High Strength #2, Batch Annealed steeland firing tests with cartridges made from this steel, were obtained byprocessing the steel in accordance with FIG. 3C. This steel has thefollowing composition: from about 0.126 to about 0.154 weight percentcarbon, from about 0.395 to about 0.605 weight percent silicon, fromabout 1.75 to about 1.95 weight percent manganese, about 0.02 weightpercent of phosphorus, about 0.005 percent sulfur, from about 0.02 toabout 0.06 weight percent aluminum, less than about 0.06 weight percentcopper, less than about 0.04 weight percent nickel, less than about 0.06weight percent chromium and less than about 0.02 weight percentmolybdenum, based on the total weight of the composition.

While specific embodiments of the invention have been described indetail, it will be appreciated by those skilled in the art that variousmodifications and alternatives to those details could be developed inlight of the overall teachings of the disclosure. Accordingly, theparticular embodiments disclosed are meant to be illustrative only andnot limiting as to the scope of the invention which is to be given thefull breadth of the appended claims and any and all equivalents thereof.

We claim:
 1. A method of processing steel of a rimfire cartridge, themethod comprising: obtaining a steel composition having an originalthickness; performing a first cold rolling of the steel composition;producing the steel composition having a reduced thickness, wherein, thereduced thickness constitutes a reduction of about 70% from the originalthickness; performing a first batch annealing and subsequent cooling ofthe steel composition having the reduced thickness; performing a secondcold rolling of the steel composition having the reduced thickness,following the first batch annealing and subsequent cooling step;producing the steel composition having an additional reduced thickness,wherein, the additional reduced thickness constitutes an additionalreduction of about 20% to about 35%; performing a second batch annealingand subsequent cooling of the steel composition having the additionalreduced thickness; and continuous plating the steel composition havingthe additional reduced thickness, following the second batch annealingand subsequent cooling step.
 2. The method of claim 1, wherein the firstand second annealing are conducted as a continuous process.
 3. Themethod of claim 1, wherein the continuous plating comprises depositingan element selected from the group consisting of zinc, brass andcombinations and alloys thereof.
 4. A method of forming a rimfirecartridge, comprising: obtaining a steel composition having an originalthickness; performing a first cold rolling of the steel composition;producing the steel composition having a reduced thickness of about 30%to 70% of the original thickness; performing a first batch annealing andsubsequently cooling of the steel composition having the reducedthickness; performing a second cold rolling of the steel compositionhaving the reduced thickness, following the first batch annealing andsubsequent cooling step; producing the steel composition having afurther reduced thickness of about 20% to 35%; performing a second batchannealing and subsequent cooling of the steel composition having thefurther reduced thickness; and continuous plating the steel compositionhaving the further reduced thickness; and forming steel compositionhaving the further reduced thickness into a rimfire cartridge.